Volume 16, Issue 5 - June/July 2015


Dew Tell: The Condensation Story
Test Standards Improve Window Efficiency, Reduce Mold and Mildew
by Dean Lewis

Just a generation ago, condensation on windows was considered an inevitable nuisance. More recently, however, the presence of moisture of all types— including condensation—is recognized as a major, yet preventable, phenomenon leading to unacceptable wall stains and moisture-induced mold growth. Furthermore, while condensation resistance does not drive energy efficiency, excessive wintertime condensation is an indicator of unacceptable thermal performance and energy waste.

How Condensation Happens

Condensation forms on a surface when the temperature of the object falls below the dew point, the temperature at which water vapor turns into a liquid. The warmer the air, the more moisture it can hold. So, as the amount of water vapor in the air (relative humidity) increases, the dew point also increases, and condensation becomes more likely. Conversely, in cold weather, the temperature of the interior surface of fenestration can fall below the dew point and encourage condensation.

For skylights, which reside in the higher parts of the building, exposure to condensation is more likely due to the tendency of both warm air (with greater capacity to hold moisture) and humid air (which is less dense than dry air) to rise to the ceiling.

The problem then becomes one of ensuring that the inside surface temperature stays above the dew point temperature when outdoor temperatures are frigid. This is the same challenge as thermal efficiency: reducing the amount of heat transfer through the window.

Predicting Condensation

In the U.S., two methods have evolved for predicting how well fenestration products resist the formation of condensation. One, developed by the American Architectural Manufacturers Association (AAMA), focuses on laboratory testing. The other, developed by the National Fenestration Rating Council (NFRC), focuses on computer simulations to make its predictions.

• The AAMA Condensation Resistance Factor

The Condensation Resistance Factor (CRF) is a numerical index. The higher the CRF rating, the greater the product’s resistance to condensation. The index generally falls in the range of 35 to 80, with 35 recommended as the minimum acceptable rating for a thermally improved product.

The CRF is derived from actual interior surface temperature readings obtained from testing per AAMA 1503-09, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections.

AAMA offers an online CRF Tool to provide general guidance on defining a target minimum CRF based on a project-specific set of environmental conditions. Visit: http://www.aamanet.org/crfcalculator/1/334/crf-tool for details.

• The NFRC Condensation Resistance Value

The NFRC’s Condensation Resistance (CR) figure is described in NFRC 500, Procedure for Determining Fenestration Product Condensation Resistance Values (March 2015). CR measures how well a product resists the formation of condensation and is based on finite-element thermal models.

CR and CRF ratings are not interchangeable, and no method exists to convert a CRF value to a CR value or vice versa. For example, a CR 50 is not equivalent to a CRF 50.

The tools available for rating fenestration systems should be used for product comparison only.

Commercial Condensation

While most of the foregoing pertains to residential situations, in commercial facilities, condensation resistance can compromise the building’s purpose and functionality. For example, in hospitals, moisture provides fertile ground for dangerous molds and bacteria and can compromise sensitive diagnostic equipment. In museums, moisture can damage priceless artworks.

AAMA 507-12, Standard Practice for Determining the Thermal Performance Characteristics of Fenestration Systems Installed in Commercial Buildings, provides a method to determine several thermal performance parameters of building-specific fenestration systems. CRF may be based on physical testing, or graphs based on one-time test results per AAMA 1503 to determine the performance of specific products. One such graph depicts ranges for the CRF. The methodology can be useful in determining overall building performance, occupant comfort and HVAC sizing and is relevant to the implementation of ASHRAE (90.1 and 189.1), IBC and IECC codes.

Dean Lewis serves as educational and technical information manager for the American Architectural Manufacturers Association in Schaumburg, Ill.


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